This application claims the priority of German Patent Application Serial No. 199 52 819.5, filed Nov. 2, 1999, the subject matter of which is incorporated herein by reference.
The present invention relates to a reflector antenna with a main reflector and a rotatable sub-reflector having a reflecting surface and located in front of the main reflector in the direction of the arriving beams. The invention is also related to a method for producing a sub-reflector for a reflector antenna.
A reflector antenna is typically used to receive electromagnetic beams emitted by a radiation source, such as a satellite, and transmit corresponding signals for amplification. The electromagnetic beams impinge on a main reflector which reflects the beams to a sub-reflector which is formed approximately at a focal point of a main antenna. The main antenna can be in the form of a dish. The sub-reflector includes a reflecting layer facing the main reflector which diverts the beams reflected by the main reflector to a receiver located in the center of the main reflector. To obtain the greatest possible cross-section of the sub-reflector, the sub-reflector rotates and is placed in the focal point of the main reflector. A typical rotation speed ranges from approximately 200 to 400 rpm. The sub-reflector is supported on a rotation axle located eccentrically relative to an axis extending through a center of the main reflector. The sub-reflector hereby scans the main reflector across a cone which opens from the sub-reflector towards the main reflector.
The eccentrically supported sub-reflector can produce undesirable vibrations which can cause the support of the sub-reflector to vibrate. The vibrations can interfere with the received signals. In a known solution for this problem, the sub-reflector rotates about its center of gravity on a rotation axis which extends substantially in the direction of the axis of the main reflector. However, the axis of the sub-reflector in this case is not parallel to the axis of the main reflector, whereby this deviation produces a vibration effect in the support of the sub-reflector. However, any vibration should not significantly affect the strength of the received signals within the rotation speed range in which the sub-reflector operates.
It is thus an object of the present invention to provide an improved reflector antenna, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved reflector antenna which is so constructed as to eliminate vibration effects.
According to one aspect of the invention, a sub-reflector has a cylindrical shaft extending in a direction parallel to a main axis of the main reflector, wherein the sub-reflector is rotatably supported on the cylindrical shaft and rotates at a high rotation speed of between approximately 1500 to 3500 pm.
With this arrangement, the main reflector is rapidly scanned by the sub-reflected so that a large number of beams are received by the sub-reflector and reflected towards the receiver. This produces a strong signal in a circuit connected to the main reflector. Although the rotation axis is oriented in the same direction as the axis of the main reflector, the hyperbolic reflector surfaces of the reflector reflect a large number of beams towards the receiver.
According to another embodiment of the invention, the sub-reflector is supported on its shaft so as to be free from vibrations. To achieve this goal, the sub-reflector has to be supported on the shaft with high precision; moreover, the shape of the sub-reflector has to be suitably selected so that no vibrations are produced in the support even at a high rotation speed.
According to another embodiment of the invention, the sub-reflector is formed as a rotating body that is free from imbalances. This is difficult to achieve mechanically, because the reflecting surface has to reflect the received beams towards the receiver as perfectly as possible. This requirement has a major impact on the shape of the reflector, which adds to the mechanical complexity imposed by the requirement that the sub-reflector has to be supported vibration-free even at high rotation speed.
According to another embodiment of the invention, the rotating body is made of a solid material that does not reflect the electromagnetic beams, with a reflecting surface embedded in the non-reflecting solid material. The non-reflecting solid material provides the rotating body with a compact form which enables a vibration-free rotation even at a high rotation speed.
According to another embodiment, the solid material has the form of a cylinder and includes two parts connected with one another, wherein one of the parts includes on the end opposite the other part the reflecting surface, with the end of the other part formfittingly fits into the reflecting surface. With this arrangement, the reflecting surface does not produce an intrinsic motion, such as a wobbling motion. The reflecting surface is fixedly connected with the non-reflecting solid material on the one hand, and acted upon by the other part as a consequence of the configuration in the form of a rigid rotating body.
According to another embodiment of the invention, a reflective coating is applied to the non-reflecting solid material for forming the reflecting surface. This layer strongly adheres to the non-reflecting solid material and does not execute an intrinsic motion, for example a wobble, even at a high rotation speed.
According to another embodiment of the invention, the reflecting layer is made of an aluminum layer that is fixedly connected with the solid material. According to another embodiment, the aluminum layer can be applied to the non-reflecting solid material by evaporation.